The present invention relates to a thin film forming method and thin film forming apparatus for forming a metal film.
As the integration degrees of integrated circuits are increasing, in a DRAM (Dynamic Random Access Memory) which is constituted by a transistor and capacitor, reduction in area of a memory cell and increase in memory capacity are required. For this request, a technique for increasing the capacity of a memory cell without increasing its area by using a material with a high dielectric constant such as tantalum oxide (Ta2O5) as a dielectric film (capacitor insulating film) constituting a capacitor has been proposed.
When a material with a high dielectric constant such as tantalum oxide is used as a dielectric film, an expected dielectric constant is obtained by performing postprocessing such as annealing, UV processing, and the like after forming a tantalum oxide film. At this time, in order to prevent oxygen from being eliminated from the dielectric material, which is an oxide, postprocessing is generally performed in an atmosphere containing oxygen. For this reason, when tungsten or titanium nitride is used as a storage electrode, it is oxidized. Accordingly, a metal material which is not easily oxidized or exhibits conductivity even after oxidization, such as ruthenium is used as the storage electrode.
In order to increase the memory capacity in a DRAM, a technique for increasing the memory capacity of a memory cell without increasing its area by forming a capacitor to have a cylinder shape or stacked electrode structure and increasing its substantial area has been proposed. For example, when a cylinder capacitor structure is used in this technique, electrodes are formed to have larger steps. For this reason, in forming a conductive film as an electrode, CVD (chemical vapor deposition), by which the conductive film can be formed with a good step coverage, is used.
In using ruthenium in thermal CVD, Ru(C2H5C5H5)2{Ru(EtCp)2} is used as a source gas, and oxygen gas is added to this source gas.
However, in forming a ruthenium film by thermal CVD, when ruthenium is not attached in advance to the underlying layer on which a film is to be formed, a ruthenium film is unlikely to grow by CVD. Conventionally, a thin ruthenium film (seed layer) is generally formed by PVD, and a ruthenium film having a desired thickness is formed with a good step coverage by CVD.
When a thin film of ruthenium is to be formed only by thermal CVD using ruthenium source gas and oxygen gas, it takes about 20 min since a substrate is heated to a predetermined temperature, and supply of the above gas onto the substrate is started until a film starts to be formed on the surface of the underlying layer. Even when the supply amount of the gas is increased to have tens of times as high concentration as usual in order to reduce time until the film starts to be formed, the film formation start time requires several min. In addition, only a film with a poor quality, e.g., a poor smoothness is formed.
To avoid such a problem, conventionally, a seed layer is first formed by PVD. However, it is difficult to form a film with a good step coverage, and thus it is very difficult to form a seed layer on the side wall of a hole with a high aspect ratio having, e.g., a diameter of 0.1 μm and depth of over 0.5 μm. When a cylinder capacitor structure is to be formed, it is required that a ruthenium film be formed with a good step coverage in a small hole with a high aspect ratio. As microminiaturization is increasingly required, it is becoming difficult to form a seed layer uniformly on a side wall.